Recent results from an ITPA joint experiment to study the onset, growth, and decay of relativistic electrons (REs) indicate that loss mechanisms other than collisional damping may play a dominant role in the dynamics of the RE population, even during the quiescent Ip flattop. Understanding the physics of RE growth and mitigation is motivated by the theoretical prediction that disruptions of full-current (15 MA) ITER discharges could generate up to 10 MA of REs with 10–20 MeV energies. The ITPA MHD group is conducting a joint experiment to measure the RE detection threshold conditions on a number of tokamaks under quasi-steady-state conditions in which Vloop, ne, and REs can be well-diagnosed and compared to collisional theory. Data from DIII-D, C-Mod, FTU, KSTAR, and TEXTOR have been obtained so far, and the consensus to date is that the threshold E-field is significantly higher than predicted by relativistic collisional theory, or conversely, the density required to damp REs is significantly less than predicted, which could have significant implications for RE mitigation on ITER.

[1] Since the early days of plasma physics research suprathermal electrons were observed to be generated during beam-plasma laboratory experiments. Energetic electrons, often modeled by distributions, are also ubiquitously observed in space. Various particle acceleration mechanisms have been proposed to explain such a feature, but all previous theories rely on either qualitative analytical method or on non-self-consistent approaches. This paper discusses the self-consistent acceleration of electrons to suprathermal energies by weak turbulence processes which involve the Langmuir/ ion-sound turbulence and the beam-plasma interaction. It is discussed that the spontaneous scatttering process, which is absent in the purely collisionless theory, is singularly responsible for the generation of distributions. The conclusion is that purely collisionless Vlasov theory cannot produce suprathermal population.

Electromagnetic radiation at the plasma frequency and/or its second harmonic, the so-called plasma emission, is widely accepted as the fundamental process responsible for solar type II and III radio bursts. There have also been occasional observations of higher-harmonic plasma emissions in the solar-terrestrial environment. This paper presents the first demonstration of multiple harmonic emission by means of twodimensional electromagnetic particle-in-cell simulation. This finding indicates that under certain circumstances the traditional mechanism of fundamental-harmonic pair emission might also be accompanied by higher-harmonic components. Consequently, the present findings are highly relevant to in situ observations of third-and/or higher-harmonic plasma emission in astrophysical and solar-terrestrial environments.

It has been known since the early days of plasma physics research that superthermal electrons are generated during beam-plasma laboratory experiments. Superthermal electrons (the kappa distribution) are also ubiquitously observed in space. To explain such a feature, various particle acceleration mechanisms have been proposed. However, self-consistent acceleration of electrons in the context of plasma kinetic theory has not been demonstrated to date. This Letter reports such a demonstration. It is shown that the collisionality, defined via the "plasma parameter" g=1/n(lambda(D)(3), plays a pivotal role. It is found that a small but moderately finite value of is necessary for the superthermal tail to be generated, implying that purely collisionless (g=0) Vlasov theory cannot produce a superthermal population.

Density profiles in pedestal region (H-mode) are measured in HL-2A and the characteristics of the density pedestal are described. Cold particle deposition by Supersonic Molecular Beam Injection (SMBI) within the pedestal is verified. ELM mitigation by SMBI into the H-mode pedestal is demonstrated and the relevant physics is elucidated. The sensitivity of the effect to SMBI pressure and duration are studied. Following SMBI, the ELM frequency increases and ELM amplitude decreases for a finite duration period. Increases in ELM frequency of SMBI ELM f / 0 ELM f 2-3.5 are achieved. This experiment argues that the ELM mitigation results from an increase in Page 2 higher frequency fluctuations and transport events in the pedestal, which are caused by SMBI. These inhibit the occurrence of large transport events which span the entire pedestal width. The observed change in the density pedestal profiles and edge particle flux spectrum with and without SMBI supports this interpretation. An analysis of the experiment and a model shows that ELMs can be mitigated by SMBI with shallow particle penetration into the pedestal.